1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for three-dimensional scanning and a method of obtaining ultrasonic images, and in particular, to a technology for adjusting the brightness of ultrasonic images.
2. Description of the Related Art
An ultrasonic imaging apparatus is capable of obtaining three-dimensional biological information and displaying a three-dimensional image by performing a 3D scan of a subject to be examined using ultrasonic waves.
On the other hand, even when an ultrasonic beam is transmitted under identical transmission conditions, the intensity of the received signal varies, depending on the orientation of the ultrasonic beam or the condition of the patient's body. For this reason, unevenness in illumination disadvantageously occurs for displaying the received signal as an ultrasonic image. Incidentally, the displayed ultrasonic image is displayed with contrasting density of brightness according to intensity of the received signal. An ultrasonic diagnosis apparatus for performing two-dimensional scans has an STC (Sensitivity Time Control) gain adjustment function for adjusting gain depthwise, or a function for adjusting gain laterally. When displaying a tomographic image that is a two-dimensional image, the brightness of the tomographic image may be adjusted by adjusting the gain depthwise or laterally (e.g., U.S. Pat. No. 6,398,733, which is incorporated herein by reference).
In addition, an ultrasonic imaging apparatus for automatically adjusting the gain for each scanning line, based on the distribution of brightness in pre-obtained ultrasonic images is known.
However, even if an attempt is made to automatically adjust the gain for each scanning line by automatically detecting the distribution of brightness in the ultrasonic image, the gain may not be appropriately adjusted. For example, when there is a highly reflective part in a short distance from the ultrasonic probe, the brightness of the short distance part becomes higher and the short distance part is brightly displayed while the brightness of the parts behind the short distance part (back regions) becomes low and the back regions are darkly displayed. When an ROI (region of interest) is placed in the back regions, it is very difficult to appropriately adjust the gain of the ROI, even if the gain is adjusted with the average value of the intensity (brightness) of the signal or with the intensity (brightness) of the signal in a predetermined depth.
For example, when observing a heart with a focus on the left ventricle by applying an ultrasonic probe to an apex, a peripheral part to be scanned becomes dark due to the ribs. For example, as the tomographic image 100 shown in FIG. 1, the ribs and surrounding cartilage are tissues through which ultrasonic waves cannot easily pass. Consequently, the ribs and cartilage become a highly reflective part (high brightness part) 101 for the ultrasonic waves, and the dark part 102 on the back may become dark.
When automatically adjusting gain, the average value of the intensity (brightness) of signals are calculated for each scanning line and the gain is adjusted for each scanning line so that the intensity (brightness) of the signal on each scanning line is uniform. However, as in the case of ribs, when a highly reflective part (high brightness part) is placed in a short distance from the ultrasonic probe, the average value of the intensity (brightness) of the signals on the scanning lines passing through that part becomes high. Therefore, it is difficult to automatically adjust it to an appropriate brightness.
When observing a tomographic image that is a two-dimensional image, an inspector can manually adjust the gain of an ROI by the STC gain adjustment or the gain adjustment laterally. However, in the case of an ultrasonic diagnosis apparatus for scanning three-dimensional space, the range for the gain adjustment is a three-dimensional space. Thus, it is difficult to specify an ROI, and consequently, the gain cannot be easily adjusted by only the STC gain adjustment and lateral gain adjustment.